This invention relates in general to sewing machines and in particular to a new and useful thread clamp for sewing machines and to a sewing machine having a needle bar containing a plurality of needles which operates with threads which is guided by a thread clamp made up of a single coil spring having spaced apart thread guiding areas.
Through U.S. Pat. No. 840,887 a thread clamp with a helical spring as a clamping element on a sewing machine is known which is fastened in an arcuate deformation form at a cloth clamping frame of the sewing machine. It is held by a holding spring in the thread pull-off path. The arrangement is such that upon removal of the work the thread comes between the turns of the helical spring and the spring expands in its arc during pull off in the one direction due to the friction of the thread between two adjacent turns and thus becomes longer. With that the brake force acting on the thread decreases, so that the work can easily be removed.
If the thread is to be cut, it must be pulled off in the opposite direction and be moved against a knife. This causes the radius of curvature of the helical spring to become flatter, its length is shortned, and the turns move closer together. Hence the brake force is increased until the thread is completely clamped before it is cut.
The needle thread and the shuttle thread each have their own helical spring. While the thread clamp manages without a drive means for opening and closing, changing the direction of the thread pull-off is not possible in most sewing operations because of an increased use of the removal and stacking devices behind the sewing station or because of interconnection with subsequent sewing stations.
Helical springs are indeed superior to thread clamps consisting of single disks or jaws movable relatively to each other because of their cost effective manufacture as a thread clamping element, but because of their relatively long opening and closing path, they have not been used until now.